The clinical success of EGFR kinase inhibitors gefitinib and eriotinib in EGFR mutant non-small cell lung cancer (NSCLC) is limited by the eventual development of acquired drug resistance. We and others have identified two main mechanisms of drug resistance: acquired secondary mutations in EGFR (EGFR T790M) and activation of HGF/MET signaling (either by MET amplification or by autocrine/paracrine production of HGF). In both instances, the resistant cancers maintain dependence on EGFR signaling, highlighting the need to develop novel EGFR-targeted therapies and/or combination therapeutic approaches. Irreversible quinazoline-based EGFR kinase inhibitors, including HKI-272, PF00299804. and B1BW2992, are effective in some preclinical models against EGFR T790M, but clinically their efficacy has been limited by lack of potency and toxicity (skin rash and diarrhea) due to concurrent on-target inhibition of wild-type (WT) EGFR. Recentiy, we identified irreversible pyrimidine EGFR kinase inhibitors (WZ4002) that are selective for the mutant EGFR receptor and are significantiy more potent against EGFR T790M than existing irreversible quinazoline EGFR inhibitors. Furthermore, these agents, unlike quinazoline irreversible EGFR inhibitors, prevent the emergence of EGFR T790M as a resistance mechanism using in vitro models. Although this new class of EGFR inhibitors may be clinically more effective than currentiy available agents, resistance is nevertheless expected to also develop to all irreversible EGFR inhibitors. In fact, a mutation in the covalent binding site (C797S) of EGFR alone is sufficient to cause resistance to both WZ4002 and HKI-272. In this project we propose to develop additional agents that can effectively inhibit EGFR in the presence of the C797S and other drug resistance mutations. The specific aims are 1.) Characterize EGFR small molecule kinase inhibitors against irreversible EGFR kinase inhibitor resistance mutations. 2.) Identify resistance mechanisms to irreversible pyrimidine EGFR inhibitors and 3.) Determine the impact of targeting drug resistance mechanisms on the efficacy of irreversible pyrimidine EGFR inhibitors and the emergence of drug resistance.